1. Field of the Invention
The invention relates to a method of processing a signal in a hearing instrument, and to a hearing instrument, in particular a hearing aid.
2. Description of Related Art
The acoustic coupling of hearing instruments to the tympanic membrane (TM/eardrum) is usually described by means of the Real-Ear-to-Coupler-Difference (RECD) that is indicative of the difference between the sound pressure at the 2 cc coupler (a standardized artificial model of the ear canal) and the sound pressure at the eardrum.
Another quantity that describes the acoustic interplay between the hearing instrument (or that part of the hearing instrument (earpiece) that is inserted into the ear) and the real ear is the Real Ear Occluded Gain (REOG). The REOG describes the sound pressure signal transfer for a sound signal incident on the ear to the eardrum when the hearing instrument is turned off.
According to prior art methods, taking into account the individual situation in a real person's ear canal requires, for example, the measurement by a microphone probe in front of the eardrum. Such a measurement is not straightforward, is time consuming and requires special equipment. Also, the protruding probe end is prone to being soiled and may cause displeasing sensations to the patient. Further, the measurement is of limited accurateness because due to the introduction of the probe, additional leakage may be caused through the earpiece. Also, the equipment would need to be calibrated to equate sensitivity variations. In addition, there are individual differences because of tubings of different lengths and variations of the earwax protection. Correction methods either require a SPICE-model fit or data stored in look-up tables. Further, estimations of the corrections are not compatible with the common Real-Ear-to-Coupler Difference (RECD).
Examples of methods to estimate the sound pressure at the eardrum by a probe that is placed not at the eardrum, but away from it are, for example, disclosed in US 2010/0202642 and in WO 2010/016925. In WO 2010/016925, the distance between the probe microphone and the eardrum is estimated based on the λ/4 resonance. The λ/4 resonance is a pronounced dip in the sound level in the ear canal. This is then used to correct the estimated sound pressure level at the TM by adding the ‘inverse of the dip function’. While these methods provide some improvement over the prior art in terms of avoiding direct probe contact to the eardrum, they still require a separate probe.
Many future hearing instruments will have an ear canal microphone for active occlusion control. Active occlusion control measures the sound level in the ear canal—by the ear canal microphone—and submits the inverse of the measured signal (filtered by a specific function) to the receiver input. US 2006/0083395 proposes to measure, by means of an ear canal microphone, the acoustic reflectance (the ratio of the reflected pressure that comes from eardrum and cochlea and of the incident pressure) for automatically adjusting processing parameters of a hearing aid. In the reflectance measurements, a measured impedance may be used as an input parameter.
However, an estimation of the sound pressure at the eardrum—and thus also of the RECD—by means of the acoustic reflection or another power quantity is not possible without further input data, especially an estimate of the ear canal cross section.
Most ear canal microphone applications are restricted to controlling the microphone sound pressure and neither comprise calibration nor an estimation of the eardrum sound pressure. Current models therefore do not appropriately consider the transfer from the ear canal microphone to the eardrum.